As compounds to regulate plant growth, for example, phytohormones are known. Phytohormones are derived from plants themselves, but phytohormones also can be synthesized as compounds and are utilized as agricultural chemicals and vitalizing agents. As such compounds to regulate plant growth, imidazole derivatives are described in Patent Literatures 1 to 4. For example, in Patent Literature 4, 7H-imidazo[4,5-d][1,2,3]triazin-4(3H)-one (another name: 2-azahypoxanthine, hereinafter, sometimes referred to as “AHX”) is described, and it is also described that AHX exhibits growth promotion or growth suppression action on plants.